Stability vs. Persistence

A recurring theme in physical organic chemistry, and especially in the study of reactive intermediates, is the notion of stability. We describe one structure as stable, but another as re- 

Similarly, it is difficult to exactly define what it means to be unstable (reactive). A molecule that spontaneously reacts all by itself at 0 degrees Kelvin in interstellar space is certainly unstable. But in a more practical discussion, instability is again always a relative term, where the compound under analysis is unstable relative to some reference system. Yet, the reference system must be studied under the same experimental conditions as the compound whose stability is being questioned, because as we now describe, the experimental conditions affect the reactivity of compounds. 

In contrast, stability is an intrinsic property of a reactive intermediate. We define a structure as stable or stabilized if it is thermodynamically (Gibbs free energy) more stable than some reference structure, as discussed above. Flere, our focus is upon electronic stability rather than sterics, because for most reactive intermediates their electronics dominates their reactivity. For example, the benzyl cation is stabilized, because it is thermodynamically more stable than its reference, the methyl cation. However, under typical conditions the benzyl cation is not expected to be persistent. Fundamentally, stability is a thermodynamic notion, while persistence is a kinetic one. Stability is intrinsic to a structure, while persistence is very much context sensitive. We will do our best to keep these distinctions clear in the following sections. In the chemical literature, however, such precision in terminology is not always maintained. 

More so than any other area of reactive intermediate chemistry, thermodynamics is a valuable predictor of radical reactivity. There will always be exceptions, and we can never totally ignore kinetics, but as a first step, we will in most instances turn to the thermochemistry of a given sifuation to predict or rationalize radical reactivity patterns. In addition, there is a vast collection of relevant thermodynamic data for radicals—the collection of BDEs discussed in Section 2.1.3. 

Consider Eq. 2.11, a simple variant of Eq. 2.5. This defines the BDE for a generic R-H bond. When comparing Eq. 2.11 for a variety of organic molecules, the contribution to 

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The issue of stability vs. persistence of free radicals is an important one that dates back to the birth of the field. In 1900 Gomberg prepared the triphenylmethyl or trityl radical according to Eq. 2.13. Under appropriate conditions, the free radical persists in solution indefinitely at room temperature. This initially controversial result was arguably the birth of reactive intermediate chemistry, and it spurred volumes of work. The trityl radical is in equilibrium with a dimer that, for decades, was assumed to be hexaphenylethane. However, nuclear magnetic resonance (NMR) and ultraviolet (UV) studies in 1968 revealed that the actual dimer was the unsymmetrical structure shown in Eq. 2.13, in which one trityl center added to the para position of a ring of another radical. 

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